Method for injection molding a golf club body

ABSTRACT

A golf club head has a golf club body which consists entirely and exclusively of a solid, unitary mass of polymethylmethacrylate (acrylic), the head being completed by a metal insert in the body which receives the end of the club shaft. The acrylic body is made by an injection molding process. The acrylic charge can be unadulterated, for producing a completely clear club body, or can be provided with a dye so that the club body can be given any desired color, while still remaining transparent. In addition to anesthetically pleasing appearance, since the club body is made out of a solid mass of material, a cleaner transfer of energy occurs from the club shaft to the face of the club head which strikes the golf ball, thereby producing a longer and more accurate drive.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a composition for, and a method formanufacturing, a golf club head, such as a driver head.

2. Description of the Prior Art

Conventional golf club driver heads consist of composite materials.So-called wood drivers actually consist of a number of differentmaterials, including a portion of the club body consisting of wood, inaddition to a plastic face, a metal heel plate, and shot weight in thecenter of the wooden portion. Metal clubs are hollow in the center andthus contain air therein.

The weight and volume of the materials which are used to produce adriver head, or any golf club head, must be calculated so that the swingweight of the club which is produced is correct for that club. This isconventionally done by adding or subtracting mass from the golf clubhead.

Examples of such golf club head consisting of a number of differentmaterials, or a single composite material, are described in U.S. Pat.Nos. 5,106,094, 4,568,088 and 3,640,534.

A putter is disclosed in U.S. Pat. No. 3,873,094 which has a part of theclub body which can consist of methylmethacrylate, or another suitableplastic material, but has weights molded therein so that the body is notexclusively composed of methylmethacrylate. A golf club having a headportion consisting primarily of wood, and being provided with an insertproviding a striking face for the golf ball is disclosed in U.S. Pat.No. 3,836,153. The insert consists of a molded piece prepared by curinga suspension of polymerized polymethylmethacrylate powder, which caninclude a few percent of a butadine styrene copolymer, in a liquidconsisting essentially of methylmethacrylate and glycolmethacrylate.

Clubs having a head portion with a bushing or sleeve which receives theend of the club shaft, and reinforces the portion of the club headaround the shaft, are disclosed in U.S. Pat. Nos. 3,640,534 and Re.16,808. Golf club heads having various ridges and depressions formed inthe outer surface of the club head are disclosed in U.S. Pat. Nos. Des.318,891 and Des. 192,515.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a golf club headwhich improves the transfer of energy from the shaft to the club facewhen striking a golf ball.

It is a further object of the present invention to provide such a golfclub head which has a unique and aesthetically pleasing appearance.

It is a further object of the present invention to provide a method formaking such a golf club head.

The above objects are achieved in accordance with the principles of thepresent invention in a golf club head having a club body which consistsentirely of polymethylmethacrylate (acrylic). The club body is formed byan injection molding process, so that the resulting club body consistsonly of one piece. Since the body consists of a single mass of material,there is a cleaner transfer of energy from the club shaft to thestriking face on the club body when swinging the club to strike a golfball.

Moreover, by making the club body out of polymethylmethacrylate, theclub body can be made completely clear, if no additives are added to thepolymethylmethacrylate charge, thereby providing the club with a uniqueappearance. If desired, however, the polymethylmethacrylate charge canbe provided with a dye additive, so as to lend the club body any desiredcolor. Even though having a color, however, the club body may stillremain transparent, although it will not be "clear" in the sense ofhaving no color at all, or it may be made opaque or translucent.

The club body can be provided with a metallic insert as part of themolding process, the insert receiving the end of the club shaft andreinforcing the club body in the region immediately surrounding theshaft. This also permits the club head to be made separately from theshaft, and later attached by inserting the shaft into the insert orbushing.

As used herein, the term "club head" means the entirety of the portionof the golf club which is attached to the club shaft, and thus includesthe metal insert or bushing. The term "club body" means the portion ofthe club head which is injection molded out of polymethylmethacrylate,and thus excludes the insert. The "club body" therefore is composedentirely and exclusively of polymethylmethacrylate, whereas the "clubhead" consists of the club body plus the metal insert.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevational view of a club head constructed andmanufactured in accordance with the principles of the present invention.

FIG. 2 is an end elevational view of the club head of FIG. 1.

FIG. 3 is a rear elevational view of the club head of FIG. 1.

FIG. 4 is a bottom view of the club head of FIG. 1.

FIG. 5 is a top view of a mold for making a club head in accordance withthe principles of the present invention by an injection molding process.

FIG. 6 is a side elevational view of one portion of the mold of FIG. 5.

FIG. 7 is a bottom view of the mold of FIG. 5.

FIG. 8 is a side elevational view of a portion of the mold of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A club head 1 constructed in accordance with the principles of thepresent invention is shown in various views in FIGS. 1 through 4. Theclub head 1 is composed of a club body 2, which consists entirely andexclusively of polymethylmethacrylate (acrylic), and a metallic insert3, which is adapted to securely receive and hold the end of a club shaft(not shown). The insert 3 may consist, for example, of aluminum. Asindicated by the shading, the club head 2 is transparent, which as usedherein means optically transmissive for normal sunlight. In oneembodiment, if no additives are added to the polymethylmethacrylate, theclub head will be completely clear, i.e., it will have no color.Suitable dyes can be added to the polymethylmethacrylate charge prior toconducting the injection molding of the club body 2, however, so thatthe club body 2 can be given any desired color, so that it may remaintransparent to light, in accordance with the above definition, or may bemade opaque or translucent.

As can also be seen in FIGS. 1 through 4, the club body 2 can beprovided with one or more depressions or "speed slots." For example, thetop of the club body 2 can be provided with speed slots 4, the end facecan be provided with speed slots 5, and the bottom of the club body 2can be provided with a speed slot 6, formed by a depression therein.

The club head 1, consisting of the club body 2 and the insert 3, canhave a total weight, for example, of 208 grams and a maximum thicknessof approximately 1.7".

Heretofore, injection molding of a mass of acrylic material of the sizecomprising the club body 2 (i.e., 200 grams or more and a thickness ofapproximately 1.7") has not been successfully accomplished withoutbubbles or defects to the extent necessary to withstand the forces towhich such a mass is subjected when striking a golf ball (i.e., a solidmass). The absence of bubbles and other defects is also necessary inorder to obtain the necessary clean transfer of energy from the clubshaft through the club body to the golf ball. Because the thickness ofthe club body is larger than polymethylmethacrylate molded productswhich are currently produced, a processing method as disclosed hereinmust be utilized in order to control volumetric shrinkage and warpage.The possibilities for such volumetric shrinkage and warpage to occur ispresent both during and after the injection molding process. Thefollowing conditions specifically contribute to volumetric shrinkage andwarpage of any large volume injection molded product, and are overcomeby the method disclosed herein for producing a club body consistingentirely and exclusively of polymethylmethacrylate.

A non-uniform pressure distribution is created when material is injectedinto the cavity of the mold at a rate and temperature which permits alarger volume of material in one section of the cavity, and lesservolume in another. This will produce a temperature differential whichproduces accompanying shrinkage and warpage in the product. Due to thethickness of the product, such shrinkage and warping in the context of agolf club body would result in the creation of voids in the center ofthe club body 2. Shrinkage will also occur which creates wrinkles on theouter surface. The difference in heat transfer to the surroundingenvironment from the outside surface of the molded body to the center ofthe molded body must be controlled to eliminate this problem.

In order to avoid the aforementioned problems giving rise to shrinkageand warpage, the process used to make the club body 2 is as follows. Thematerial used to form the club body 2 may be, for example, Rohm and HaasVO45 Acrylic. The injection molding press which is used to create theclub body 2 from a charge of polymethylmethacrylate must be large enoughto support a 12 oz. shot. A 150-200 ton machine will, for example, besufficient. Commercially available injection molding machines includethe capability of controlling a number of temperature zones in the pressbarrel. In an injection molding machine having four such temperaturezones, the first zone is preferably set at 410° F., the second zone ispreferably set at 400° F., the third zone is preferably set at 390° F.,and the fourth zone (tip) is preferably set at 390° F.

The injection molding machine must also have the capability of profilingthe injection. This is a known function of injection molding machineswhich permits an increase or decrease in the pressure needed to fill andpack the cavity at any stage of the filling process. Filling takes placein a first stage of the process. The profile for the club body head 2will require an increase in pressure during the first stage starting at30% pressure from 0 to 50% of fill. The profile will steadily increaseto 80% pressure from 50 to 80% of fill. The final first stage pressureis 900-1,260 psi. This will reduce or eliminate the amount of shrinkageby increasing the density of the fill.

Increased second stage pressure of 900-2,000 psi is then applied to holdand pack the material in the cavity to retain the aforementioned densityuntil gate freeze-off. The time at which this increased pressure in thesecond stage must be maintained should be in the range of 3 to 4minutes, or more. A typical total cycle time will consist of a firststage injection of 15-35 seconds, a hold and pack stage lasting from 3to 4 minutes, followed by 4 to 5 minutes of cooling time.

The club body 2 will at this point be ejected from the mold, and willimmediately be placed in a secondary cooling bath of water at 180°-200°F. and/or hot oil at 180°-200° F. This secondary bath is important forcontrolling heat transfer during cooling of the body, because the bodywill be ejected from the mold with a minimum wall thickness of 0.125" to0.1875". The center of the body part will still be molten. The secondarybath is used to prolong the extraction of heat from the body, and tostabilize the difference (gradient) in temperature from the center ofthe club body to the surface of the club body. The body will remain inthe bath for 40 to 60 minutes. The body is then extracted from the bath,and is buffed and polished.

Examples of molds used to produce the club body 2 are shown in FIGS.5-8. The overall mold 7 consists of two separable parts 8 and 9. Themold 7 is provided with cooling lines drilled straight through each ofthe sides 8 and 9, with a 0.125" offset of the holes facing each otherin each side 8 and 9, so as to produce turbulence. The coolant flowingin the respective cooling lines controls the temperature in a zonesurrounding each cooling line. The coolant is not circulated through allthe lines together, but instead each line forms its own closed loop, sothat each line can be individually controlled with a temperaturecontroller (not separately shown) connected to each of the coolinglines. Oil at 170°-220° F. is suitable as the coolant.

The mold 7 has a cold runner 10 with a cold slug 11 and a cold gate 18,and a sprue bushing 39 in which the hot tip gate (not shown) will bedisposed. Examples of placement of the cooling lines in the mold part 8are as follows. The cooling line 12 can be placed 0.6000" measuredperpendicularly from the surface 8a of the part 8, and 5.875" from aplane extending through the center line of the cold slug 11perpendicular to the mold seam between the parts 8 and 9. The coolingline 14 may be 1.4" from the surface 8a and 4.950 from the cold slugplane. The cooling line 15 may be 2.000" from the surface 8a and 4.125"from the cold slug plane. The cooling line 15 may 2.450" from thesurface 8a and 3.125" from the cold slug plane. The cooling lines 16 and17 may each be 0.875" from the surface 8b of the part 8, and disposed at1.000" on opposite sides of the cold slug plane. The associated coolinglines 19, 20, 21, 22, 23 and 24 in the part 9 will be disposed at thesame distances from the surfaces 8a and 8b as the aforementioned lines,but each will be offset from the facing cooling line by 0.125" so that,for example, the cooling line 19 may be at 6.000" from the cold slugplane, the cooling line 20 may be 5.075" from the cold slug plane, etc.The center lines of the cooling lines 25 through 30 in the lower portionof the part 8 may be disposed at the following locations. The coolingline 25 may be disposed at a distance of 1.600" from a plane containingthe surface 8c and 5.750" from the cold slug plane. The cooling line 26may be disposed at a distance of 2.600" from the plane containing thesurface 8c and 4.750" from the cold slug plane. The cooling line 27 maybe disposed also at 2.600" from the plane containing the surface 8c and3.375" from the cold slug plane. The cooling line 28 can be disposed2.000" from the plane containing the surface 8c and 2.125" from the coldslug plane. The cooling line 29 can be disposed 1.100" from the planecontaining the surface 8c and 1.125" from the cold slug plane. Thecooling line 30 can also be disposed 1.100" from the plane containingthe surface 8c, and at 1.190" on the opposite side of the cold slugplane. The cooling lines 31, 32, 33, 34 and 35 in the part 9 will bedisposed at the same distances from the plane containing the surface 8c,but offset from the facing cooling line by 0.125".

The tool will be equipped with a hot tip gate (not separately shown).The gate size must be in excess of 0.25" in hydraulic diameter toprovide good flow through the gate for an adequate period of time beforefreeze-off occurs.

Processing conditions must be tightly controlled so as not to createexcessive head in the cavity. The following processing parameters arepreferable.

Injection Temperature: 380°-410° F.

Mold Temperature: 180°-220° F.

Oil Coolant in Tool: 190°-220° F.

Water Temperature (bath): 180°-200° F.

Injection Time: 15-35 seconds

As shown in FIG. 6, the mold 7 is provided with a sleeve 37 which,during the injection molding process, receives a steel post 38 on whichthe insert or bushing 3 will be placed prior to each filling of the mold7. The insert 3 is held in place within the cavity during the moldingprocess by the post 38. The insert must be heated to 500° F. to preventcold shock of hot plastic. The acrylic flows around the insert 3 to holdit in place in the finished product. As can be seen in FIG. 3, theinsert 3 is provided at its exterior with a plurality of channels, and asuitable knurling such as a criss-cross pattern of ridges is arranged inthose channels, around which the molten acrylic flows, and subsequentlyhardens to hold the insert in place in the club body 2.

Although modifications and changes may be suggested by those skilled inthe art, it is the intention of the inventors to embody within thepatent warranted hereon all changes and modifications as reasonably andproperly come within the scope of their contribution to the art.

I claim as our invention:
 1. A method for injection molding a golf clubbody comprising the steps of:filling a mold cavity withpolymethylmethacrylate; varying pressure acting on saidpolymethylmethacrylate in said cavity during filling of said cavityuntil reaching a first pressure; holding said polymethylmethacrylate insaid cavity at a second pressure higher than said first pressure for aperiod of approximately 3-4 minutes; providing a selected temperatureprofile in said mold surrounding said cavity during holding of saidpolymethylmethacrylate in said cavity at said second pressure; coolingsaid polymethylmethacrylate in said cavity for a period of approximately4-5 minutes; ejecting said polymethylmethacrylate from said cavity inthe form of a molded body; and cooling said molded body in a hot oilbath at 180°-200° F. after ejection for a period in the range of 20 to40 minutes to produce a golf club body consisting entirely of a solidmass of polymethylmethacrylate.
 2. A method as claimed in claim 1wherein the step of varying pressure during filling of said cavity withsaid polymethylmethacrylate is further defined by increasing pressure to30% of said first pressure during 0-50% of filling of said cavity, andincreasing said pressure to 80% pressure from 50 to 80% of filling ofsaid cavity.
 3. A method as claimed in claim 2 wherein the step offilling said cavity with polymethylmethacrylate is further defined byfilling said cavity with polymethylmethacrylate during a period in therange of 15 to 35 seconds.
 4. A method as claimed in claim 1 wherein thestep ofproviding a selected temperature profile in said mold surroundingsaid cavity is further defined by arranging a plurality of cooling linesin said mold around said cavity at selected locations and controlling aflow of coolant through said cooling lines during holding of saidpolymethylmethacrylate in said cavity and during cooling of saidpolymethylmethacrylate in said mold and cooling said molded body in acooling bath after ejection for a period in the range of 20 to 40minutes to produce a golf club body consisting entirely of a solid massof polymethylmethacrylate.
 5. A method as claimed in claim 1 whereinsaid first pressure is in a range of 900-1200 psi and said secondpressure is in a range of 900-2000 psi.
 6. A method for injectionmolding a golf club body comprising the steps of:filling a mold cavitywith polymethylmethacrylate; varying pressure acting on saidpolymethylmethacrylate in said cavity during filling of said cavityuntil reaching a first pressure by increasing pressure to 30% of saidfirst pressure during 0-50% of filling of said cavity and increasingsaid pressure to 80% pressure from 50 to 80% of filling of said cavity;holding said polymethylmethacrylate in said cavity at a second pressurehigher than said first pressure for a period of approximately 3-4minutes; providing a selected temperature profile in said moldsurrounding said cavity during holding of said polymethylmethacrylate insaid cavity at said second pressure; cooling said polymethylmethacrylatein said cavity for a period of approximately 4-5 minutes; ejecting saidpolymethylmethacrylate from said cavity in the form of a molded body;and cooling said molded body in a cooling bath after ejection for aperiod in the range of 20 to 40 minutes to produce a golf club bodyconsisting entirely of a solid mass of polymethylmethacrylate.
 7. Amethod as claimed in claim 6 wherein the step of filling said cavitywith polymethylmethacrylate is further defined by filling said cavitywith polymethylmethacrylate during a period in the range of 15 to 35seconds.
 8. A method as claimed in claim 6 wherein the step of coolingsaid molded body in a bath after ejection is further defined by coolingsaid molded body in a water bath at 180°-200° F.
 9. A method as claimedin claim 6 wherein the step of cooling said molded body in a bath afterejection is further defined by cooling said molded body in a hot oilbath at 180°-200° F.
 10. A method as claimed in claim 6 wherein the stepof providing a selected temperature profile in said mold surroundingsaid cavity is further defined by arranging a plurality of cooling linesin said mold around said cavity at selected locations and controlling aflow of coolant through said cooling lines during holding of saidpolymethylmethacrylate in said cavity and during cooling of saidpolymethylmethacrylate in said mold.
 11. A method as claimed in claim 6wherein said first pressure is in a range of 900-1200 psi and saidsecond pressure is in a range of 900-2000 psi.
 12. A method forinjection molding a golf club body comprising the steps of:filling amold cavity with polymethylmethacrylate; varying a pressure acting onsaid polymethylmethacrylate in said cavity during filling of said cavityuntil reaching a first pressure in a range of 900-1200 psi; holding saidpolymethylmethacrylate in said cavity at a second pressure in a range of900-2000 psi for a period of approximately 3-4 minutes; providing aselected temperature profile in said mold surrounding said cavity duringholding of said polymethylmethacrylate in said cavity at said secondpressure; cooling said polymethylmethacrylate in said cavity for aperiod of approximately 4-5 minutes; ejecting saidpolymethylmethacrylate from said cavity in the form of a molded body;and cooling said molded body in a cooling bath after ejection for aperiod in the range of 20 to 40 minutes to produce a golf club bodyconsisting entirely of a solid mass of polymethylmethacrylate.
 13. Amethod as claimed in claim 12 wherein the step of cooling said moldedbody in a bath after ejection is further defined by cooling said moldedbody in a water bath at 180°-200° F.
 14. A method as claimed in claim 12wherein the step of cooling said molded body in a bath after ejection isfurther defined by cooling said molded body in a hot oil bath at180°-200° F.
 15. A method as claimed in claim 12 wherein the step ofvarying pressure during filling of said cavity with saidpolymethylmethacrylate is further defined by increasing pressure to 30%of said first pressure during 0-50% of filling of said cavity, andincreasing said pressure to 80% pressure from 50 to 80% of filling ofsaid cavity.
 16. A method as claimed in claim 15 wherein the step offilling said cavity with polymethylmethacrylate is further defined byfilling said cavity with polymethylmethacrylate during a period in therange of 15 to 35 seconds.